A polymer film which is capable of exhibiting piezoelectric properties is prepared by stretching it to establish a preferred axis and then by subjecting the film to an electric field oriented transversely through the major surfaces of the film. This causes an average net rotation of molecular dipoles within the material with the dipoles being rotated toward alignment with the electric field.
Thereafter, when a sheet of this material is subjected to an electric field transversely and preferably perpendicularly through its major surfaces, the dipoles tend to be rotated. This rotation causes a strain to occur principally along the stretch direction. If the later field which is used to control the strain is in the same direction as the field utilized to initially align the dipoles, then the material is shortened. If the field is opposed to the initial poling field, then it is opposed to the existing dipoles and causes an elongation strain in the direction of the stretch.
The controlling electric field may be most conveniently applied by forming a conductive film on each of the exposed, opposite major surfaces of the piezoelectric polymer. This is conveniently done by applying conductive paint or depositing a metalized surface of nickel or aluminum, for example, on those major surfaces.
Piezoelectric films such as polyvinylidene fluoride (PVDF), are particularly useful in applications in which short stroke and/or small angular motion is required. For example, as applied to mechanical actuation, 28 micron thick poled PVDF film at an excitation level of 10 volts per micron (280 V) and with an active length of 1.0 meter, yields a theoretical displacement along the material stretch axis of 230 microns. This displacement can be arranged to produce an angular displacement of 6.6 degrees when operated over a radius of 2.0 millimeters.